The cellulose membranes which are subsequently subjected to the repeated drying-rewetting procedure can be made by any of the common techniques. Thus, regenerated cellulose made by the cuprammonium process and known as cuprophane and regenerated cellulose made by the xanthate process and known as celluphane can be employed. Recently, additional solvents have been found which are capable of dissolving cellulose. These solvents include ammonium thiocyanate in liquid ammonia, lithium chloride in dimethyl acetamide, N-methyl-morpholine-N-oxide, dinitrogen tetroxide/dimethyl formamide and dimethylsulfoxide/paraformaldehyde. Cellulose membranes made relying on these solvent systems can also be used in the present invention.
The lithium chloride/dimethyl acetamide solvent is particularly useful for membrane preparation since cellulose solutions which are very stable and do not degrade can be prepared at room temperature. When using that solvent system a gelation solvent of tetrahydrofuran at 2.degree. C. is preferred (see Kobunshi Ronbunshu 41 (9) 501-9, 1984; also see JP No. 60-187307 Sept. 24, 1985)
It is known that drying regenerated cellulose membrane in the absence of any swelling or softening agent causes collapse of the internal structure of the membrane by causing intra chain hydrogen bonding. See U.S. Pat. No. 4,496,456. This can result in an increase in the membrane's rejection behavior toward particular feed components under reverse osmosis conditions. This drying usually is for up to 48 hours at up to 60.degree. C. and causes an increase in rejection and a decrease in flux. The percent rejection reported typically increases until it reaches a steady value. Further increases of either the drying temperature or time do not lead to further increases in rejection.
It is known that regenerated cellulose can be used under several osmosis conditions to separate extraction solvents such as NMP from raffinate or extract oil under reverse osmosis conditions. Such a process is disclosed and claimed in U.S. Pat. No. 4,510,047.
However, the literature does not suggest the effect repeated membrane drying and rewetting will have on membrane performance, i.e., selectivity/rejection and flux, for such separations.